Conventionally, mark display units for ship navigation, which display on a display screen a mark associated with a position, are known. JP1996-020267B and JP1993-288562A disclose this kind of mark display units for ship navigation.
The mark display unit for ship navigation of JP1996-020267B displays a mark associated with a position where various information (e.g., fishing data) is assigned, on a display screen along with a trace, so as to add information to an important location (e.g., fishing spot).
The ship trace display unit of JP1993-288562A designs multiple marks according to a purpose of use of the display unit, and displays the marks in various manners.
JP2923252B discloses a ship electronic instrument including a rotary indicator and a push button incorporated in the rotary indicator. Although JP2923252B does not refer to anything about displaying a mark, the ship electronic instrument of JP2923252B selects one of adjusting functions by the push button, and adjusts a to-be-adjusted amount corresponding to the selected adjusting function by the rotary indicator.
However, with each of the configurations of JP1996-020267B and JP1993-288562A, when the number of marks stored in the mark display unit for ship navigation is increased and a large amount of information of marks accumulates, a nautical chart, a radar image, a trace, etc. displayed on the display screen may be covered by the marks and cannot be seen, or the marks may be displayed densely and the individual marks may be difficult to visually be recognized. Therefore, there are rooms for improvements in these regards.